The present invention relates to an apparatus which is used for measuring the radial force at various locations along the circumference of a tire assembled on a wheel rim to determine the variations therein so that a tire and rim assembly having an area exhibiting an unsatisfactory variation in radial force from the rest of the assembly, which may cause unacceptable vehicle vibrations, may be detected and compensated for.
When a tire is out of round (with a 0.001 inch run-out being equivalent to approximately four pounds of radial force) due to non-uniform internal construction or due to an out-of-round rim, the tire, when rolling along a road surface, will exhibit a significant variation in the radial force existing at the ground contact patch, the circumferential area of the tire contacting the flat road surface at a particular point in time. In the worst cases, such variations in radial force may be in the range of 400 pounds of variation for a single tire, such as truck tire, while tires on passenger cars may exhibit a considerably lower variation in radial force measurements.
Since many trucks have dual tires on each rear axle wheel end, dual tires that are mounted so that their radial force variations are additive will impart a very high degree of vibration into the vehicle.